The present invention relates generally to semiconductor devices, and more specifically, to semiconductor laser diodes.
Semiconductor laser diodes are constructed from an electrical p-n junction and a quantum well. The electrical p-n junction has an n-side and a p-side. The quantum well is provided between the n-side and the p-side. Electrons from the n-side and holes from the p-side of the p-n junction recombine in the quantum well, which results in an emission of laser light. The n-side and the p-side of the p-n junction form an optical waveguide for the emitted laser light.
Semiconductor laser diodes generate heat during their operation. The heat generated is removed by an external cooling system. For a fixed level of laser light output, the energy consumed by the external cooling system is inversely proportional to the power conversion efficiency of a semiconductor laser diode. The power conversion efficiency of the semiconductor laser diode is proportional to the rate of recombinations of the electrons from the n-side and the holes from the p-side in the quantum well and inversely proportional to the operating voltage of the semiconductor laser diode.
The operating voltage of a semiconductor laser diode depends mainly on three factors: (1) the lasing wavelength of the semiconductor laser diode, (2) the resistance of the semiconductor laser diode, and (3) the band offsets of the different material systems that are used to form the n-side and the p-side of the semiconductor laser diode.
In order to produce a desired lasing wavelength, the electrons at the quantum well require an energy, which is hc/λ above the energy of the holes, where h is Planck's constant, c is the speed of light in the laser diode medium, and λ is the wavelength at which the laser is being operated. Since the energy difference hc/λ for a given wavelength is determined by the basic physical parameters, the operating voltage of the semiconductor laser diode cannot be lowered by lowering the energy difference hc/λ.
The resistance of the semiconductor laser diode depends on factors such as, the mobility of the electrons and the holes within the semiconductor laser diode and the doping levels of the material systems that are used to form the n-side and the p-side of the semiconductor laser diode. The doping levels of the material systems used to form the n-side and the p-side of the semiconductor laser diode may be selected to reduce the overall resistance of the semiconductor laser diode. Further, the doping levels of the material systems are selected such that excessive absorption of light within the semiconductor laser diode is avoided.
The electrons on the n-side and the holes on the p-side require energy to move into the quantum well. This energy is equal to the band offset of the material system used to form the semiconductor laser diode. The band offset of the material system is measured relative to the band energy of the substrate. An external voltage source provides the energy required by the electrons and the holes. The external voltage provided is proportional to the band offset.
Existing semiconductor laser diodes have the n-side and the p-side formed from a single material system. Examples of the material system include Aluminum Gallium Arsenide (AlGaAs) and Indium Gallium Arsenic Phosphide (InGaAsP). The substrate of the semiconductor laser diode is formed from a first material system that is different from the material system used to form the n-side and the p-side. Examples of the first material system include Gallium Arsenide (GaAs) and Indium Phosphide (InP). Any given material system will have a characteristic set of valence band and conduction band offsets calculated relative to the first material system, which affect both the turn-on voltage for the semiconductor laser diode and the efficiency of confining the electrons and the holes within the quantum well. An ideal material system should have a small valence band offset for the holes and a large conduction band offset for the electrons on the p-side of the semiconductor laser diode. At the same time, the ideal material system should have a large valence band offset for the holes and a small conduction band offset for the electrons on the n-side of the semiconductor laser diode. These competing band offset conditions may not be achieved by using a single material system. Therefore, a semiconductor laser diode formed from a single material system has a higher turn-on voltage and a lower charge carrier confinement at the quantum well. High power conversion efficiency requires low turn-on voltages and good charge carrier confinement.
In view of the foregoing discussion, there is a need to improve the power conversion efficiency of the existing semiconductor laser diodes. In addition, there is a need to lower the turn-on voltage for the existing semiconductor laser diodes while maintaining good charge carrier confinement at the quantum well.